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The intraocular lens may be inserted at the time of lens extraction or as a secondary procedure. Rigid implant lenses are made from perspex. Foldable lenses made from silicone or acrylic allow the advantages of a small wound to be preserved. The image size is virtually normal; sight can be restored sooner following the operation; and there are no problems with daily cleaning and removal. Three types of implant may be used:
The main difficulty is calculating the strength of lens required. This is assessed pre-operatively from measurements of the axial length of the eye - using ultrasonography - and the curvature of the cornea. The calculation is an estimate and subsequent correction may be necessary.
The main disadvantage with the intra-ocular lens is the risk of damage to corneal endothelium resulting in oedema of the corneal stroma - bullous keratopathy. This is particularly high in anterior chamber lenses and is why they tend to be reserved for patients undergoing intracapsular surgery or when the posterior capsule has been inadvertently ruptured during extracapsular surgery.
The incidence of bullous keratopathy is lower with posterior chamber lenses. However, they may cause opacification of the retained posterior capsule of the lens. Also, they may prevent the pupil from being widely dilated and so are contra-indicated in patients with retinal disease e.g. past history of retinal detachment, diabetes with retinopathy (1)